Potentiometer



Oct. 9, 1962 c. R. LAUBENFELS 3,058,080

POTENTIOMETER Filed Feb. 20. 1961 INVEN TOR. CLARE/VC'' I?. AUJE/VFE'LS ATTORNEYS'.

United States Patent O 3,058,080 PUTENTEMETER Clarence R. Laubeniels, 333 Scholl Drive, Glendale, Calif. Filed Feb. 20, 1961, Ser. No. 90,462 8 Claims. (Cl. 3138-143) This invention relates to a potentiometer.

An object of this invention is to provide a structure and means for assembling the structure whereby a device is produced whose performance equals or exceeds that of expensive precision potentiometers, at a cost comparable to that of ordinary potentiometers.

A related object of the invention is to provide a rotary potentiometer without backlash, with low hysteresis, good repeatability, high resolution, and high linearity. In attempting to attain these objectives, precision potentiometers become very expensive, and their designs impose undesirable limitations on the device produced. For example, in a potentiometer a relatively high resistance is desirable, but there is an inherent limitation imposed by the way conventional precision potentiometers have been built and assembled. The usual ten-turn precision potentiometer in a 7/s inch diameter size ordinarily has a maximum standard resistance of about 125,000 ohms, and this is about as great as can be attained. To illus- .trate the advantages of the instant invention, a 7% inch diameter precision potentiometer made according to it can be produced with a maximum resistance of 300,000 ohms. As to the other characteristics above mentioned, `it provides approximately twice .the performance of previously known, commercially available precisie-n potentiometers.

A potentiometer according to this invention comprises a frame having a bore yformed by an interior wal-l, the bore having a central axis. A helical groove is formed in this wall, the helical groove forming a means for receiving a helical winding, and also forming a helical track between adjacent convulutions of the groove for a rotor to ride on.

The rotor has an exterior thread by which it is engaged to the helical track. Preferably the rotor makes an interference fit With the track, and this is possible when W-friction materials are used, thereby eliminating play in the rotor. The rotor has a central non-circular opening which receives a shaft. The shaft is also non-circular, so that the shaft and rotor are in a torque-transmitting, but axially slidable relationship to each other.

The rotor carries a brush which moves along the winding as the rotor is turned and advanced along the helical track.

According to a prefer-red but optional feature of the invention, the frame is expansible, such as by being slit, enabling the edges of the slit to be pried apart, or by providing it in a plurality of iframe sections which can be assembled together. This enables the frame to be assembled around the winding, so that the winding is placed in the groove in its relaxed condition without any distortion along its length.

The above and other `features of this invention will be fully understood from the following detailed description and the accompanying drawings in which:

FIG. l is a cross-section of the presently preferred embodirnent of lthe invention;

FIG. 2 is a left-hand end view of FIG. l;

FIG. 3 is a cross-section taken at line 3-3 of FIG. l; and

FIG. 4 is a cross-section of an alternate embodiment of frame for use in the invention.

A potentiometer 10 according to the invention is shown in FIG. l. It is assembled within an exterior cylindrical case 11 (which may be tubular as shown). The case 3,058,080 Patented Oct. 9, 1962 is fitted with end plates l2, 13. End plate 12 carries three terminals 14, 15, 16. Terminals 14 and 15 are coil terminals, and terminal 16 is a brush terminal. Insulating inserts 17, 18, 19 are fitted in holes in the end plate to pass the terminal leads and insulate them from the end plate.

A splined shaft 20l is journaled in the two end plates. It is made in three sections: Spline section 2.1 with splines 22, 23 extending parallel to the axis 24 of the device, connecting pin section 25, and drive section Z6. The spline section and drive section are made of metal to resist Wear. Also, the spline section should be electrically conductive. The connecting pin is made of insulation material to insulate the splined section from the outside of the potentiometer except through terminal 16. An insulating spacer 27 is placed between the spline section and the drive section. It bears against right-hand end plate 13.

A rotor 30 (FIG. 3) comprises a disc having an exterior thread 31 which extends peripherally around the rotor, except for a cut-out portion 32. The thread thus extends around the major portion of the periphery, and, with minor modifications could extend around lthe full periphery. A brush 33 has one end attached to the cut-out portion of the rotor. Its -free end 34 rides the interior surface of a winding 35. A Whisker 36 makes electrical contact with brush 33 and bears against the splined shaft.

A second Whisker 37 rides the left-hand end of the shaft and constitutes a portion of terminal 16. It will now be seen that when the shaft is turned, the rotor will turn, carrying with it brush 33, an electrical connection thereby being made between the Winding and terminal 16. Note that only the litee end of the brush rides the winding. The rotor is supported and guided by the case, and not the winding. Any lost motion of the brush and any appreciable wear on the winding are eliminated.

The case encloses a frame 40 which is made of two frame sections 4l, 4Z, each of which includes a concavely formed section 43, 44, respectively. Neither of the sections rforms more than of the interior wall of the bore which is formed by assembling them together. Each of the frame sections includes a fragment 45, 46 of a continuous groove, which when the case sections are assembled together, `form a continuous helical groove. Between the groove sections, there are formed track sections 47, 48, which when assembled `form a continuous helical track to which the thread 3l of the rotor is engaged. The extreme edges of the cut-out portion are slanted to eliminate abrupt shoulders adajacent to the thread which might interfere with rotation when the edges encounter the split edges of the frame.

FIG. 4 shows a frame embodiment which can be used in place of frame 40. Frame 60 comprises a piece of tubing 61 with an internal helical groove 62 and helical track 63. An axial slit 64 is formed without removal of material. The material of the frame is sufficiently flexible that the slit can be pried open.

When the winding is placed in either embodiment of the frame, and the frame is put in the casing, the helical track is continuous, and preferably the rotor makes an yinterference fit with it. Ordinarily this would make for too great a drag, but this is avoided by making the rotor of a low-friction material, such as Teflon. This totally eliminates play between the rotor and the track, and corrects one of the greatest deficiencies in conventional potentiometers: play in the system between the shaft, brush and winding.

The preferred embodiment of shaft is a splined one. The term splined is not intended to be restrictive to mean a circular shaft having axial, rectangular flanges. It comprehends all generally prismatic shapes, which might be such as triangular, square, or polygonal. The

shaft shown does, however, have significant advantages. The circular portion `of the shaft is smaller than the circular portion of the rotor opening, and the two splines are defined by a pair of parallel planes. Also, the splines are shorter than the corresponding part of the rotor opening. The shaft is thereby rendered self-aligning. ln addition, the driving faces make an interference fit, so that there is no lost rotational motionbetween shaft and rotor. This tight lit is tolerable because the area of contact is small, and low-friction material is used. Thus there is no substantial frictional drag against axial movement of the rotor along the shaft.

While the invention is useful for many types of potentiometers, its principal use is in connection with precision potentiometers. The device shown is a ten-turn device (7/s O.D.) which can be made with va total resistance as high as 300,000 ohms, which is much higher than attai-nable with conventional potentiometers. A winding suitable for use with it is conveniently formed by taking an initially straight piece of insulated wire 49 :and Wrapping a coil 50 of resistance wire around it and then forming the coil-wound wire 49 into a helix. Then to assemble the device of FIG. 1, this helix in its relaxed condition is first lowered into one of the frame sections. Then the other yframe section is placed over it, few drops of cement placed between the two frame sections, if desired. Then the case is slid over the assembled case sections to hold them together. The rotor, shaft, and end plates are put in position, the internal electrical connections are made, and the potentiometer is ready for use. When the frame of FIG. 4 is used, the slit is pried open to admit the winding.

The advantages of the aforesaid technique of assembly and the construction described which makes it possible will be best appreciated by comparison with previously used methods of assembling conventional precision potentiometers. In the past, the frame was internally grooved, and the wire-wound resistance element in helix form was screwed into the frame. This, of course, caused distortion during the insertion, resulting in errors of linearity. As can easily be understood, more end Aforce must be exerted on the helix to get it in the last turn than at the rst turn which results in uneven `forces exerted over the length of it so that the helix is not loaded down uniformly over its whole length in the groove. In addition, the previously known methods of rotating the contact have required considerable clearance between the parts, resulting in back lash which shows up as a hysteresis error and results in non-repeating readings. All of this is avoided by means of either of the construction shown, whereby in FIG. l, a completely relaxed winding is placed between frame sections, and the frame sections are then xed together, or in FIG. 4, where the frame is opened to admit a relaxed winding. With either technique, there are no end `forces at all'exerted on the helix so that the helix condition is unifonm from end to end, thereby overcoming the aforesaid problems. In addition, there are no problems of clearance as to getting .the helix in becanse the lit in this device can be an exact one.

As to the accuracy of the contact mounting, it will be seen that in FIG. l the rotor rides a continuous helical thread and because it overlaps more than 180 of the periphery, keeps the case sections in proper alignment. In FIG. 4, the materialsprings back to alignment. Furthermore, when the low `friction material, such as Teilon, is used, the fit between the rotor and the track can be an interference lit so that there is no tolerance or spacing of any kind which would permit random motion of the contact. Thus, the device shown is totally Without assembly errors and without errors which would result' from the necessity of providing clearances throughout the device. This is the reason i hat this invention provides a precision potentiometer whose performance-characteristics are at least twice as good as any of those known in the prior art, and it will be seen thatthe assembly technique is very much simpler. This also results in economies, because the spoilage rates in previous precision potentiometers have been quite high due to the distorted Iforces which had to be exerted on the helix when the device was put together.

This invention thereby provides a simple device capable of Ibeing readily and inexpensively assembled, and which provides exceptional performance in a potentiometer which costs no more than conventional potentiometers.

This invention is not to be limited by the embodiment shown in the drawings and description which is given by way rof example and not of limitation, but only in accordance with the scope of the appended claims.

I claim:

l. A potentiometer comprising: a plurality of frame sections, each having a concavely for-med section; a tragment of a continuous helical groove in each trame section, said groove forming a fragment of a continuous helical track, said frame sections being assembled to form a cylindrical bore, each section forming no more than of the interior wall of said bore and forming a continuous interior helical groove and track, all having a common central axis; a helical winding Seated within the groove between adjacent convolutions of the track; a rotatable splined shaft in said bore on the central axis; a rotor having a splined central opening and an exterior thread, the splined opening making a driving connection with the splined shaft, and the thread riding the helical track; and a contact carried by the rotor riding the winding, whereby driving the shaft 1rotates .the rotor to advance it along the track, and with it, to advance the contact along the winding, the rotor making no contact with the winding.

2. A potentiometer according to claim l in which each end of the cylindrical bore is closed by an end plate, the splined shaft' being journaled in the plates.

3. A potentiometer according to claim 2 in which the `splined shaft is made of three sections: a splined section bearing the splines; a drive section outside the bore; and a connecting pin section made of insulating material joining the pin section to the drive section.

4. A potentiometer comprising: a plurality of frame sections, each having a concavely-formed section; a fragnient of a continuous helical groove in each frame section, said groove forming a fragment of a continuous helical track, said frame sections being assembled to form a cylindrical bore, each section forming no more than 180 of the interior wall of said bore and forming a continuous interior helical groove and track, all having a common central axis; 4an exterior case surrounding and embracing the frame sections to hold them assembled; a pair of end plates, one at each end of the cylindrical bore; a helical winding seated within the groove between adjacent convolutions of the track; ya rotatable splined sha-ft in said bore on the central axis and journaled in the two end plates; a rotor having a splined central opening and an exterior thread, the splined opening making a driving connection ywith the splined shaft, and the thread riding the helical track; and a contact carried by the rotor riding the winding, whereby the shaft rotates the rotor to advance it along the track, and with it, to advance the contact along the winding, the rotor making no contact with the Winding.

5. A potentiometer according to claim 4 in which the rotor has a cut-out portion for the Contact, said cut-out portion having slanted edges adjacent the thread to eliminate sharp shoulders which might interfere with rotation by engaging the edges of the frame.

6. A potentiometer laccording to claim 4 in which the exterior thread on the rotor extends substantially around the full perimeter of the rotor.

7. A potentiometer according to claim 4 in which the winding has a pair of terminals mounted to one of the end plates and in which arthir-d terminal is connected to a brush which rides the splined shaft and in which the contact is electrically connected to the splined shaft, the

splined shaft' being made of conductive material whereby said third terminal 'acts as a brush connection to the potentiometer winding.

8. A potentiometer according to claim 4 in which the rotor is made of material having a low coeicient of friction, and the rotor makes an interference lit with the helical track.

2,815,422 Lock Dec, 3, 1957 2,850,606 MacLaren Sept. 2, 1958 2,900,614 Gach et al Aug. 18, 1959 

